Optically variable devices are used in a wide variety of applications, both decorative and utilitarian; for example, such devices are used as security devices on commercial products. Optically variable devices can be made in numerous ways to achieve a variety of effects. Examples of optically variable devices include holograms imprinted on credit cards and authentic software documentation, color-shifting images printed on banknotes and enhancing the surface appearance of items such as motorcycle helmets and wheel covers.
Optically variable devices can be made as a film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigment is commonly called color-shifting pigment because the apparent color of images appropriately printed with such pigments changes with the change of the angle of view and/or illumination. A common example is the numeral “20” printed with color-shifting pigments in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Optically variable devices can also be made with magnetic pigments that are aligned with a magnetic field. After coating a product with a liquid composition, a magnet with a magnetic field having a desirable configuration is placed on the underside of the substrate. Magnetically alignable flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original orientation. This tilt varies from normal to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflect incident light back to the viewer, while the reoriented flakes do not.
A variety of methods have been suggested for forming images and security device which include magnetically aligned pigment flakes.
U.S. Pat. No. 5,630,877 in the name of Kashiwagi et al. discloses placing shaped magnets underneath a substrate and spraying the substrate with a paint containing magnetic particles. The resulting images are formed by narrow contour lines outlining the shapes of the magnets where the field lines bend.
U.S. Pat. No. 7,047,883 in the name Raksha et al. discloses alignment of magnetic particles, dispersed in organic binder and coated onto a substrate, between two poles of a horseshoe magnet or between north and south poles of two separated magnets 194, 196 as illustrated in FIG. 1. The magnets 194, 196 create magnetic field 192 with force lines 198 that are essentially parallel to the substrate 29, the magnetic field causes magnetic pigment flakes 26 in a fluid carrier 28 to flatten out.
WO2011092502 in the name of Bargir et al. discloses an apparatus (FIG. 2A) which includes a housing 13 placed inside of a block 15. The housing 13 has a curved upper surface 13a and a cavity 13b wherein a permanent magnet 12 is installed and covered with a magnetizable sheet 11. The magnet 12 is shaped such that its lateral periphery has the form of indicia; in FIG. 2A the magnet is a sphere. The sheet 11 acts as a focusing element for the magnetic field and concentrates the perturbations into the immediate lateral vicinity of the permanent magnet within the layer 20′ as illustrated in FIG. 2B. This leads to a very sharp and well defined visual appearance of the indicia.
EP1990208 in the name of Gygi et al. discloses magnetic transfer of indicia to a coating composition P (FIG. 3A), such as an ink or varnish comprising magnetic pigments, applied to a sheet S. The device includes a body 20 exposed to a magnetic field generated by two permanent magnets 31 and 32. The body 20 consists of a support 22 and a shaped metal peace 21 engraved with a desirable pattern 21a-21c. The magnetic pigments align along the field lines and produce the appearance of engraved characters, such as a dark numeral in middle of a shiny oval of a security device shown in FIG. 3B. FIG. 3C illustrates the security device printed on a banknote.
The aforedescribed methods provide security patches unrelated to the graphical design of underlying documents in the sense that the patches may be placed anywhere on the document or transferred from one document to another. There is a security risk associated with possible transfer of a patch to a forged document. Accordingly, there is a need to mitigate the disadvantages of existing security patches and provide a new method of forming images including magnetically aligned pigment particles.